1. Field of the Invention
The present invention relates to a camera provided with an automatic focusing device, and more particularly to a movie camera or a television camera provided with a rangefinding optical system for automatic focusing independently from the picture taking lens.
2. Description of the Prior Art
Cameras with automatic focusing function, particularly movie cameras with such function, have been commercialized in recent years, but these cameras are invariably associated with significantly larger dimensions than in the conventional cameras. As already known, the automatic focusing method employed in the presently commercialized automatic focusing movie cameras is based on the same working principle as in the conventional double-image matching range-finder, utilizing two incident beams guided to photodetector elements through two mirrors of which one is movable, and a focusing signal is generated in response to the time required for the matching of two image corresponding to the distance to the object for controlling a switch to be actuated in cooperation with the displacement of the focusing lens in the picture-taking system. Such automatic focusing camera will therefore require, in addition to the mechanisms of the conventional camera, an automatic focus detecting unit consisting of an automatic focusing optical system and photodetector elements, a motor for driving the mirror in the range-finding optical system, a drive mechanism for picture-taking focusing lens, a drive motor therefore, a mechanism for detecting the amount of displacement of the focusing lens, electric circuits for controlling the automatic control etc., of which addition inevitably results in the increase of dimensions, thus deteriorating the handling characteristics of such automatic focusing movie cameras.
In another method the range-finding mechanism and the focusing lens are mechanically separated but are linked and controlled through comparison of electric signals, as disclosed in the copending U.S. Patent Applications Ser. Nos. 944,974 and 121,690 respectively filed Sept. 22, 1978 and Feb. 15, 1980. Such method is advantageous, due to the absence of a mechanical link between the range-finding mechanism and the focusing lens, in allowing independent adjustment and providing a larger freedom in the layout. Such method however requires signal generating means for indicating the distance at which the focusing lens is adjusted. For this purpose a potentiometer is usually used as an encoder coupled with rotation of the focusing lens, but such solution cannot be considered ideal in consideration of the precision, cost and space required.
Still another solution lies on the use of a digital switch board composed of code patterns provided on a printed circuit board corresponding to the number of addresses required for representing the focus distance of the picture taking lens in a desired precision from infinity to the closest focus distance in combination with a brush sliding over said code patterns in response to the displacement of the focusing lens, and such method is considered most preferable for use in ordinary movie cameras in consideration of the achievable cost and precision. In such a digital switch board the number of steps of distance divisions from the infinity to the closest focus distance depends on the precision required by the taking lens, and for example requires 5-bit code patterns for 32-step distance divisions.
In general, such digital switch is preferably formed as a circular encoder positioned close to the picture taking lens. However, in a movie camera or a television camera, a larger lens tube diameter than the camera body thickness is not desirable from the practical and esthetic point of view, and, for this reason, it becomes difficult to position a flat-shaped digital switch in the vicinity of the picture taking lens. It would also be possible to mount a tubular printed circuit board having such code patterns around the lens tube, but the preparation of inexpensive code patterns having satisfactory surface smoothness and abrasion resistance has not been achieved so far.